Vehicle Steering Device

ABSTRACT

A vehicle steering device includes a driver torque estimation unit that includes a driver torque estimation observer, a low-pass filter, and a hands-on/off determination unit. The driver torque estimation observer estimates driver torque applied to a steering wheel by a driver on the basis of a detected value that includes at least torsion bar torque detected by a torque sensor and the rotational angle of an electric motor detected by a rotational angle sensor. The low-pass filter performs a low-pass filtering process on the estimated driver torque. The hands-on/off determination unit determines whether in a hands-on state or a hands-off state on the basis of the driver torque after being subjected to the low-pass filtering process.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2015-252324 filed onDec. 24, 2015 including the specification, drawings and abstract, isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a vehicle steering device.

2. Description of the Related Art

Japanese Patent Application Publication No, 2004-175122 (JP 2004-175122A) discloses an electric power steering system (EPS) that includes anelectronic control unit (ECU) that includes a function of determiningthe steering wheel operation state. The ECU described in JP 2004-175122A computes the product of a time differential value dθh/dt of a steeringangle θh detected by a steering angle sensor and torque Tp detected by atorque sensor as a work load W on the steering wheel, and determines thesteering wheel operation state using the work load W. Specifically, inthe case where the work load W exceeds a predetermined threshold W1(W1>0) in the positive direction, or in the case where the work load Wexceeds a predetermined threshold −W1 in the negative direction, the ECUdetermines an actual steering state in which a driver is operating thesteering wheel.

In the case where the absolute value of the work load W is equal to orless than the threshold W1 and the absolute value of the torque Tp isequal to or less than a predetermined threshold Tp1 (Tp1>0), the ECUdetermines a state in which the steering wheel is not held, In the casewhere the absolute value of the work load W is equal to or less than thethreshold W1 and the absolute value of the steering angle θh is equal toor less than a predetermined threshold θh1 (θh1>0), the ECU determines astate in which the steering wheel is held.

In the electric power steering system described in JP 2004-175122 A, ifan external force (load torque) is input from a road surface when theactual steering wheel operation state indicates that the steering wheelis not held, the torque Tp and the steering angle are varied, whichgenerates the work load W. Therefore, in the related art, it may beerroneously determined that the steering wheel is actually operated evenif the steering wheel is not held. In the related art, in addition, thework load W crosses zero when the steering wheel is operated in theopposite direction even if the actual steering wheel steering stateindicates that the steering wheel is actually operated. Therefore, itmay be erroneously determined that the steering wheel is not held duringsuch a time.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a vehicle steeringdevice capable of precisely determining whether in a hands-on state inwhich a driver is grasping a steering wheel or a hands-off state inwhich the driver is not grasping the steering wheel.

An aspect of the present invention provides a vehicle steering deviceincluding:

-   a steering wheel;-   an input shaft to which the steering wheel is coupled;-   an output shaft coupled to the input shaft via a torsion bar;-   an electric motor coupled to the output shaft via a speed reduction    mechanism;-   a torque detector that detects torsion bar torque applied to the    torsion bar;-   a rotational angle detector that detects a rotational angle of the    electric motor; and-   a steering wheel operation state determination device.-   The steering wheel operation state determination device includes:

a driver torque estimation observer that estimates driver torque appliedto the steering wheel by a driver using a detected value that includesat least the torsion bar torque detected by the torque detector and therotational angle of the electric motor detected by the rotational angledetector;

a low-pass filter that performs a low-pass filtering process on thedriver torque estimated by the driver torque estimation observer; and

a hands-on/off determination unit that determines whether in a hands-onstate or a hands-off state on a basis of the driver torque after beingsubjected to the low-pass filtering process by the low-pass filter.

With this configuration, driver torque is estimated by the driver torqueestimation observer. A low-pass filtering process is performed on theestimated driver torque. Consequently, it is possible to preciselyestimate the driver torque applied to the steering wheel by the driver.Whether in a hands-on state or a hands-off state is determined on thebasis of the driver torque after being subjected to the low-passfiltering process. Thus, it is possible to precisely determine whetherin a hands-on state or a hands-off state.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further features and advantages of the invention willbecome apparent from the following description of example embodimentswith reference to the accompanying drawings, wherein like numerals areused to represent like elements and wherein:

FIG. 1 is a schematic diagram illustrating a schematic configuration ofan electric power steering system (EPS) according to an embodiment ofthe present invention;

FIG. 2 is a block diagram illustrating the electric configuration of anelectronic control unit (ECU);

FIG. 3 is a block diagram illustrating the electric configuration of asteering wheel operation state determination unit;

FIG. 4 is a schematic diagram illustrating the configuration of aphysical model of a column-type EPS;

FIG. 5 is a block diagram illustrating the configuration of a drivertorque estimation unit;

FIG. 6 illustrates s a e transition for explaining operation of ahands-on/off determination unit;

FIG. 7A is a graph illustrating the test result for a paved road;

FIG. 7B is a graph illustrating the test result for a bad road; and

FIG. 8 is a block diagram illustrating another example of theconfiguration of the driver torque estimation unit.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 is a schematic diagram illustrating a schematic configuration ofan electric power steering system (EPS) according to an embodiment ofthe present invention,

An electric power steering system (vehicle steering device) 1 is anelectric power steering system of a column assist type (hereinafterreferred to as a “column type EPS”) in which an electric motor and aspeed reduction mechanism are disposed in a column portion.

The column-type EPS 1 includes a steering wheel 2 that serves as asteering member used to steer a vehicle, a steering mechanism 4 thatsteers steered wheels 3 in conjunction with rotation of the steeringwheel 2, and a steering assist mechanism 5 that assists a driver insteering. The steering wheel 2 and the steering mechanism 4 aremechanically coupled to each other via a steering shaft 6, a firstuniversal joint 28, an intermediate shaft 7, and a second universaljoint 29.

The steering shaft 6 includes an input shaft 8 coupled to the steeringwheel 2, and an output shaft 9 coupled to the intermediate shaft 7 viathe first universal joint 28. The input shaft 8 and the output shaft 9are coupled so as to be rotatable relative to each other on the sameaxis via a torsion bar 10. That is, when the steering wheel 2 isrotated, the input shaft 8 and the output shaft 9 are rotated in thesame direction while being rotated relative to each other.

A torque sensor 11 is provided around the steering shaft 6. The torquesensor 11 detects torsion bar torque Ttb applied to the torsion bar 10on the basis of the amount of relative rotational displacement betweenthe input shaft 8 and the output shaft 9. The torsion bar torque Ttbdetected by the torque sensor 11 is input to an electronic control unit(ECU) 12.

The steering mechanism 4 is composed of a rack-and-pinion mechanism thatincludes a pinion shaft 13 and a rack shaft 14 that serves as a steeredshaft. The steered wheels 3 are coupled to end portions of the rackshaft 14 via tie rods 15 and knuckle arms (not illustrated). The pinionshaft 13 is coupled to the intermediate shaft 7 via the second universaljoint 29. The pinion shaft 13 is rotated in conjunction with a steeringoperation of the steering wheel 2. A pinion 16 is coupled to the distalend of the pinion shaft 13.

The rack shaft 14 extends linearly along the right-left direction (whichis orthogonal to the straight travel direction) of the vehicle. A rack17 meshed with the pinion 16 is formed at an intermediate portion of therack shaft 14 in the axial direction. The pinion 16 and the rack 17convert rotation of the pinion shaft 13 into movement of the rack shaft14 in the axial direction. The steered wheels 3 can be steered by movingthe rack shaft 14 in the axial direction.

When the steering wheel 2 is operated (rotated), rotation of thesteering wheel 2 is transferred to the pinion shaft 13 via the steeringshaft 6 and the intermediate shaft 7. The pinion 16 and the rack 17convert rotation of the pinion shaft 13 into movement of the rack shaft14 in the axial direction. Consequently, the steered wheels 3 aresteered.

The steering assist mechanism 5 includes an electric motor 18 thatgenerates a steering assist force, and a speed reduction mechanism 19that amplifies and transfers output torque of the electric motor 18 tothe steering mechanism 4. In the embodiment, the electric motor 18 is athree-phase brushless motor. The speed reduction mechanism 19 iscomposed of a worm gear mechanism that includes a worm gear 20 and aworm wheel 21 meshed with the worm gear 20. The speed reductionmechanism 19 is housed in a gear housing 22 that serves as a transfermechanism housing. In the following description, the speed reductionratio (gear ratio) of the speed reduction mechanism 19 is occasionallyrepresented by N. The speed reduction ratio N is defined as a ratioωwg/ωww of an angular velocity ωwg of the worm gear 20 to an angularvelocity www of the worm wheel 21.

The worm gear 20 is rotationally driven by the electric motor 18. Theworm wheel 21 is coupled so as to be rotatable together with the outputshaft 9. The worm wheel 21 is rotationally driven by the worm gear 20.

When the driver operates the steering wheel 2, the worm gear 20 isrotationally driven by the electric motor 18. Consequently, the wormwheel 21 is rotationally driven, which rotates the steering shaft 6(output shaft 9) along with applying motor torque to the steering shaft6. Rotation of the steering shaft 6 is transferred to the pinion shaft13 via the intermediate shaft 7. Rotation of the pinion shaft 13 isconverted into movement of the rack shaft 14 in the axial direction.Consequently, the steered wheels 3 are steered. That is, the worm gear20 is rotationally driven by the electric motor 18 to enable steeringassist by the electric motor 18.

Torque applied to the speed reduction mechanism 19 includes motor torqueapplied by the electric to 18 and external torque other than the motortorque. The external torque other than the motor torque includes drivertorque Tsw applied to the steering wheel 2 by the driver, and loadtorque (road load) applied to the rack shaft 14 (speed reductionmechanism 19) from the steered wheels 3.

The rotational angle of a rotor of the electric motor 18 is detected bya rotational angle sensor 25 such as a resolver. The vehicle speed isdetected by a vehicle speed sensor 26. An output signal from therotational angle sensor 25 and a vehicle speed V detected by the vehiclespeed sensor 26 are input to the ECU 12. The electric motor 18 iscontrolled by the ECU 12.

FIG. 2 is a schematic diagram illustrating the electric configuration ofthe ECU 12.

The ECU 12 includes a microcomputer 40, a drive circuit (three-phaseinverter circuit) 31 controlled by the microcomputer 40 so as to supplyelectric power to the electric motor 18, and a current detection unit 32that detects a current (hereinafter referred to as a “motor current”)that flows through the electric motor 18.

The microcomputer 40 includes a central processing unit (CPU) and amemory (such as a read-only memory (ROM), a random-access memory (RAM),and a non-volatile memory), and executes a predetermined program tofunction as a plurality of function processing units. The plurality offunction processing units includes a motor control unit 41 and asteering wheel operation state determination unit 42.

The motor control unit 41 achieves appropriate steering assist thatmatches the steering condition by controlling drive of the drive circuit31 on the basis of the vehicle speed V detected by the vehicle speedsensor 26, the torsion bar torque Ttb detected by the torque sensor 11,the rotational angle of the electric motor 18 and the motor currentdetected by the current detection unit 32, the rotational angle of theelectric motor 18 computed on the basis of the output from therotational angle sensor 25.

Specifically, the motor control unit 41 sets a current command value,which is a target value for the motor current which flows through theelectric motor 18, on the basis of the torsion bar torque Ttb and thevehicle speed V. The current command value corresponds to a target valuefor a steering assist force (assist torque) that matches the steeringcondition. The motor control unit 41 controls drive of the drive circuit31 such that the motor current detected by the current detection unit 32becomes closer to the current command value.

The steering wheel operation state determination unit 42 determines, onthe basis of the torsion bar torque Ttb detected by the torque sensor 11and the rotational angle of the electric motor 18 computed on the basisof the output from the rotational angle sensor 25, whether in a hands-onstate in which the driver is grasping the steering wheel or a hands-offstate in which the driver is not grasping the steering wheel.

FIG. 3 is a block diagram illustrating the electric configuration of thesteering wheel operation state determination unit 42.

The steering wheel operation state determination unit 42 includes adriver torque estimation unit 51, a low-pass filter 52, and ahands-on/off determination unit 53. The driver torque estimation unit 51estimates the driver torque Tsw on the basis of the output signal fromthe rotational angle sensor 25 and the torsion bar torque Ttb detectedby the torque sensor 11. The low-pass filter 52 performs a low-passfiltering process on the driver torque Tsw estimated by the drivertorque estimation unit 51. The hands-on/off determination unit 53determines whether in a hands-on state or a hands-off state on the basisof driver torque Tsw′ after being subjected to the low-pass filteringprocess by the low-pass filter 52. Such processes will be describedbelow.

The driver torque estimation unit 51 estimates the driver torque Tswapplied to the steering wheel 2 by the driver using a physical model ofthe column-type EPS.

FIG. 4 is a schematic diagram illustrating the configuration of aphysical model of a column-type EPS. The entirety of FIG. 4 represents atwo-inertia-system model M2 of the column-type EPS. A portion of FIG. 4indicated by the dashed line represents a one-inertia-system model M1 ofthe column-type EPS.

The one-inertia-system model M1 includes the steering wheel. The drivertorque (steering wheel torque) Tsw is input to the steering wheel.

The two-inertia-system model M2 includes the steering wheel and a lowercolumn. The lower column includes an assist motor, the worm gear, andthe worm wheel. The worm gear and the worm wheel constitute the speedreduction mechanism. The driver torque Tsw is input to the steeringwheel. Torque corresponding to a value N·Tms obtained by multiplyingmotor torque Tms by the speed reduction ratio N of the speed reductionmechanism and load torque Tls applied to the lower column from thesteered wheels are input to the lower column.

The symbols used in FIG. 4 are defined as follows.

-   Jsw: steering wheel inertia-   Tsw: driver torque-   Ttb: torsion bar torque-   ktb: torsion bar rigidity-   ctb: torsion bar viscosity-   N: speed reduction ratio-   θsw: steering wheel angle-   dθsw/dt: steering wheel angular velocity-   Jeq: lower column inertia-   θww: worm wheel angle-   dθww/dt: worm wheel angular velocity-   Tls: load torque (reverse input torque)

In the embodiment, the driver torque estimation unit 51 uses theone-inertia-system model M1, and estimates the driver torque Tsw using adriver torque estimation observer (disturbance observer, extendeddisturbance observer).

The equation of motion for the steering wheel inertia of theone-inertia-system model M1 is represented by the following formula (1).

J _(sw){umlaut over (θ)}_(sw) =T _(sw) −k _(tb)(θ_(sw)−θ_(ww))−c_(tb)({dot over (θ)}_(sw)−{dot over (θ)}_(ww))  (1)

“d²θsw/dt²” is the acceleration of the steering wheel.

The equation of state for the one-inertia-system model M1 is representedby the following formula (2).

$\begin{matrix}\left\{ \begin{matrix}{{\overset{.}{\hat{x}}e} = {{{Ae}\hat{x}e} + {{Beu}\; 1} + {{Le}\left( {y - \hat{y}} \right)}}} \\{\hat{y} = {{{Ce}\hat{x}s} + {{Deu}\; 1}}}\end{matrix} \right. & (2)\end{matrix}$

“̂xe” (xe with a hat) in the formula (2) is a state variable vector, andis represented by the following formula (3).

$\begin{matrix}{{\hat{x}s} = \begin{bmatrix}{\theta \; {sw}} \\{\overset{.}{\theta}\; {sw}} \\{Tsw}\end{bmatrix}} & (3)\end{matrix}$

“u1” in the formula (2) is an input vector, and is represented by thefollowing formula (4).

$\begin{matrix}{{u\; 1} = \begin{bmatrix}{\theta \; {ww}} \\{\overset{.}{\theta}\; {ww}}\end{bmatrix}} & (4)\end{matrix}$

“y” in the formula (2) is an output vector (measured value), and isrepresented by the following formula (5). “̂y” in the formula (2) is anestimated value of the output vector.

y=T _(tb) =k _(tb)(θ_(sw)−θ_(ww))  (5)

“Ae” in the formula (2) is a system matrix, and is represented by thefollowing formula (6).

$\begin{matrix}{{Ae} = \begin{bmatrix}0 & 1 & 0 \\\frac{- {ktb}}{Jsw} & \frac{- {ctb}}{Jsw} & \frac{1}{Jsw} \\0 & 0 & 0\end{bmatrix}} & (6)\end{matrix}$

“Be” in the formula (2) is an input matrix, and is represented by thefollowing formula (7).

$\begin{matrix}{{Be} = \begin{bmatrix}0 & 0 \\\frac{ktb}{Jsw} & \frac{- {ctb}}{Jsw} \\0 & 0\end{bmatrix}} & (7)\end{matrix}$

“Le” in the formula (2) is an observer gain matrix, and is representedby the following formula (8).

$\begin{matrix}{{Le} = \begin{bmatrix}{G\; 1} \\{G\; 2} \\{G\; 3}\end{bmatrix}} & (8)\end{matrix}$

“G1”, “G2”, and “G3” in the formula (8) are observer gains, and havebeen. set in advance.

“Ce” in the formula (2) is an output matrix, and is represented by thefollowing formula (9).

C_(e) =[k _(tb)OO]  (9)

“De” in the formula (2) is a direct matrix, and is represented by thefollowing formula (10).

D _(e) =[−k _(tb) O]   (10)

The driver torque estimation unit 51 computes the state variable vectorAxe on the basis of the equation of state represented by the formula(2). Consequently, an estimated value of the driver torque Tsw isobtained.

FIG. 5 is a block diagram illustrating the configuration of the drivertorque estimation unit 51.

The driver torque estimation unit 51 includes a worm wheel anglecomputation unit (θww computation unit) 61, a worm wheel angularvelocity computation unit 62, and a driver torque estimation observer101. The driver torque estimation observer 101 includes an input vectorinput unit 63, a direct matrix multiplier 64, an output matrixmultiplier 65, a first adder 66, a gain multiplier 67, an input matrixmultiplier 68, a system matrix multiplier 69, a second adder 70, anintegrator 71, and a state variable vector output unit 72.

The worm wheel angle computation unit 61 computes a rotational angle(hereinafter referred to as a “rotor rotational angle”) θm of an outputshaft of the electric motor 18 on the basis of the output signal fromthe rotational angle sensor 25, and computes a rotational angle(hereinafter referred to as a “worm wheel angle”) θww of the worm wheel21 on the basis of the obtained rotor rotational angle θm. Specifically,the worm wheel angle θww is computed by dividing the rotor rotationalangle θm by the speed reduction ratio N of the speed reduction mechanism19.

The worm wheel angular velocity computation unit 62 computes a wormwheel angular velocity dθww/dt by differentiating the worm wheel angleθww, which is computed by the worm wheel angle computation unit 61, withrespect to the time.

The driver torque estimation observer 101 estimates the driver torqueTsw on the basis of the worm wheel angle θww computed by the worm wheelangle computation unit 61, the worm wheel angular velocity dθww/dtcomputed by the worm wheel angular velocity computation unit 62, and thetorsion bar torque Ttb detected by the torque sensor 11.

The worm wheel angle θww computed by the worm wheel angle computationunit 61 and the worm wheel angular velocity dθww/dt computed by the wormwheel angular velocity computation unit 62 are provided to the inputvector input unit 63. The input vector input unit 63 outputs the inputvector u1 (see the formula (4)).

The output from the integrator 71 is the state variable vector ̂xe (seethe formula (3)). When the computation is started, an initial value ofthe state variable vector ̂xe is provided. The initial value of thestate variable vector ̂xe is zero, for example.

The system matrix multiplier 69 multiplies the state variable vector ̂xeby the system matrix Ae. The output matrix multiplier 65 multiplies thestate variable vector ̂xe by the output matrix Ce (see the formula (9)).The direct matrix multiplier 64 multiplies the input vector u1 outputfrom the input vector input unit 63 by the direct matrix De (see theformula (10)).

The first adder 66 subtracts the output (Ce·̂xe) from the output matrixmultiplier 65 and the output (De·u1) from the direct matrix multiplier64 from the output vector (measured value) y which is the torsion bartorque Ttb detected by the torque sensor 11. That is, the first adder 66computes the difference (y−̂y) between the output vector y and theestimated value of the output vector ̂y (=Ce·̂xe+De·u1). The gainmultiplier 67 multiplies the output (y−̂y) from the first adder 66 by theobserver gain Le (see the formula (8)).

The input matrix multiplier 68 multiplies the input vector u1 outputfrom the input vector input unit 63 by the input matrix Be (see theformula (7)). The second adder 70 computes a differential value d̂xe/dtof the state variable vector by adding the output (Be·u1) from the inputmatrix multiplier 68, the output (Ae·̂xe) from the system matrixmultiplier 69, and the output (Le(y−̂y)) from the gain multiplier 67. Theintegrator 71 computes the state variable vector ̂xe by integrating theoutput (d̂xe/dt) from the second adder 70. The state variable vectoroutput unit 72 outputs the steering wheel angle (estimated value) θsw,the steering wheel angular velocity (estimated value) dθsw/dt, and thedriver torque (estimated value) Tsw on the basis of the state variablevector ̂xe. The driver torque (estimated value) Tsw output from thestate variable vector output unit 72 is provided to the low-pass filter52 (see FIG. 3).

Returning FIG. 3, the low-pass filter 52 attenuates a frequencycomponent of the driver torque Tsw from the driver torque estimationunit 51 that is higher than a predetermined cut-off frequency fc. In theembodiment, the low-pass filter 52 is a second-order Butterworth filter.The driver torque Tsw′ after being subjected to the low-pass filteringprocess by the low-pass filter 52 is provided to the hands-on/offdetermination unit 53.

FIG. 6 illustrates state transition for explaining operation of thehands-on/off determination unit 53.

In the description of operation of the hands-on/off determination unit53, the driver torque Tsw after being subjected to the low-passfiltering process by the low-pass filter 52 is referred to simply asdriver torque Tsw′.

The hands-on/off determination unit 53 distinguishes four states, namelya hands-on state (ST1) with the driver torque more than a threshold, ahands-on state (ST2) with the driver torque equal to or less than thethreshold, a hands-off state (ST3) with the driver torque equal to orless than the threshold, and a hands-off state (ST4) with the drivertorque more than the threshold, as the state of a steering wheeloperation by the driver.

In the hands-on state (ST1) with the driver torque more than thethreshold, the absolute value of the driver torque Tsw′ is more than apredetermined threshold α(>0).

In the hands-on state (ST2) with the driver torque equal to or less thanthe threshold, the absolute value of the driver torque Tsw′ is equal toor less than the threshold α.

In the hands-off state (ST3) with the driver torque equal to or lessthan the threshold, the absolute value of the driver torque Tsw′ isequal to or less than the threshold α.

In the hands-off state (ST4) with the driver torque more than thethreshold, the absolute value of the driver torque Tsw′ is more than thepredetermined threshold α.

The threshold α is set to a value within the range of 0.1 [Nm] or moreand 0.3 [Nm] or less, for example.

When the absolute value of the driver torque Tsw′ is more than thethreshold α at the time of start of computation, the hands-on/offdetermination unit 53 determines that the steering wheel operation stateis the hands-on state (ST1) with the driver torque more than thethreshold. The hands-on/off determination unit 53 sets an output signal(out) to one, and sets a time counter value hod_timer to zero. Theoutput signal (out) is a signal that represents the determinationresult. When the output signal (out) is one, the determination resultrepresents hands-on. When the output signal (out) zero, thedetermination result represents hands-off.

When the absolute value of the driver torque Tsw′ becomes equal to orless than the threshold α in the hands-on state (ST1) with the drivertorque more than the threshold, the hands-on/off determination unit 53determines that the steering wheel operation state has become thehands-on state (ST2) with the driver torque equal to or less than thethreshold. The hands-on/off determination unit 53 sets the output signal(out) to one. In the case where the hands-on state (ST2) with the drivertorque equal to or less than the threshold is determined, thehands-on/off determination unit 53 updates the time counter valuehod_timer to a value obtained by adding a predetermined time Ts [sec] tothe current value (hod_timer) each time Ts elapses.

When the absolute value of the driver torque Tsw′ becomes more than thethreshold α before the time counter value hod_timer reaches apredetermined hands-off determination threshold β (>0) in the hands-onstate (ST2) with the driver torque equal to or less than the threshold,the hands-on/off determination unit 53 determines that the steeringwheel operation state has become the hands-on state (ST1) with thedriver torque more than the threshold, and sets the time counter valuehod_timer to zero.

When the time counter value hod_timer reaches the hands-offdetermination threshold β without the absolute value of the drivertorque Tsw′ becoming more than the threshold α in the hands-on state(ST2) with the driver torque equal to or less than the threshold, thehands-on/off determination unit 53 determines that the steering wheeloperation state has become the hands-off state (ST3) with the drivertorque equal to or less than the threshold. The hands-on/offdetermination unit 53 sets the output signal (out) to zero, and sets thetime counter value hod_timer to zero. The hands-off determinationthreshold β is set to a value within the range of 0.5 [sec] or more and1.0 [send] or less, for example.

When the absolute value of the driver torque Tsw′ becomes more than thethreshold α in the hands-off state (ST3) with the driver torque equal toor less than the threshold, the hands-on/off determination unit 53determines that the steering wheel operation state has become thehands-off state (ST4) with. the driver torque more than the threshold.The hands-on/off determination unit 53 sets the output signal (out) tozero. In the case where the hands-on state (ST4) with the driver torquemore than the threshold is determined, the hands-on/off determinationunit 53 updates the time counter value hod_timer to a value obtained byadding the predetermined time Ts [sec] to the current value (hod_timer)each time Ts elapses.

When the absolute value of the driver torque Tsw′ becomes equal to orless than the threshold α before the time counter value hod_timerreaches a predetermined hands-on determination threshold γ (>0) in thehands-off state (ST4) with the driver torque more than the threshold,the hands-on/off determination unit 53 determines that the steeringwheel operation state has become the hands-off state (ST3) with thedriver torque equal to or less than the threshold, and sets the timecounter value hod_timer to zero. The hands-on determination threshold γis set to a value within the range of 0.05 [sec] or more and 0.1 [sec]or less, for example.

When the time counter value hod_timer reaches the hands-on.

determination threshold γ without the absolute value of the drivertorque Tsw′ becoming equal to or less than the threshold α in thehands-off state (ST4) with the driver torque more than the threshold,the hands-on/off determination unit 53 determines that the steeringwheel operation state has become the hands-on state (ST1) with thedriver torque more than the threshold. The hands-on/off determinationunit 53 sets the output signal (out) to one, and sets the time countervalue hod_timer to zero.

When the absolute value of the driver torque Tsw′ is equal to or lessthan the threshold α at the time of start of computation, thehands-on/off determination unit 53 determines that the steering wheeloperation state is the hands-off state (ST3) with the driver torqueequal to or less than the threshold. The hands-on/off determination unit53 sets the output signal (out) to zero, and sets the time counter valuehod_timer to zero.

In the embodiment, the driver torque Tsw is estimated using the drivertorque estimation observer (disturbance observer, extended disturbanceobserver). A high-frequency component of the estimated driver torque Tswis removed. A hands-on/off determination is made using the torquethreshold α, and the time counter value hod_timer on the basis of thedriver torque Tsw′ after removal of the high-frequency component.Therefore, it is possible to precisely determine whether in a hands-onstate in which a driver is grasping a steering wheel or a hands-offstate in which the driver is not grasping the steering wheel.

The hands-on/off determination result can be utilized for mode switchingcontrol in a vehicle that has an automatic operation mode and a manualoperation mode as operation modes, such as switching to the manualoperation mode after confirming that the hands-on state has beenestablished when switching is made from the automatic operation mode tothe manual operation mode, for example.

The effect of the cut-off frequency of the low-pass filter 52 on thedetermination precision was examined. Specifically, the torsion bartorque Ttb detected by the torque sensor 11 when the vehicle istraveling on a paved road at a vehicle speed of 20 [km/h] with thesteering wheel at the neutral position was measured chronologically.Such a measurement was made in each of the hands-on state and thehands-off state. in addition, the torsion bar torque Ttb detected by thetorque sensor 11 when the vehicle is traveling on a bad road(stone-paved road) at a vehicle speed of 20 [km/h] with the steeringwheel at the neutral position was measured chronologically. Such ameasurement was made in each of the hands-on state and the hands-offstate.

This experiment was conducted for the purpose of examining the effect ofthe cut-off frequency of the low-pass filter 52, Therefore, the acquiredchronological data on the torsion bar torque Ttb were considered as thedriver torque. A low-pass filtering process was performed on theacquired chronological data on the torsion bar torque Ttb using aplurality of types of low-pass filters with different cut-offfrequencies. A desired threshold was set to determine hands-on in thecase where the value of the chronological data after the low-passfiltering process was equal to or more than the threshold, and todetermine hands-off in the case where the value of the chronologicaldata after the low-pass filtering process was less than the threshold.

A false positive rate and a true positive rate at the time when thethreshold was varied from zero to a sufficiently large value werecalculated. The false positive rate refers to the percentage [%] atwhich the steering wheel operation state determination unit erroneouslydetermined events that were actually hands-off as the hands-on state.The true positive rate refers to the percentage [%] at Which thesteering wheel operation state determination unit correctly determinedevents that were actually hands-on as the hands-on state. Suchcalculations were performed for each of the cases where the cut-offfrequency fc of the low-pass filter was 1 [Hz], 3 [Hz], 5 [Hz], 7 [Hz],and 9 [Hz].

FIG. 7A is a graph illustrating the test result for a paved road. FIG.7B is a graph illustrating the test result for a bad road. In FIGS. 7Aand 7B, the horizontal axis represents the false positive rate, and thevertical axis represents the true positive rate. Each of the curves inFIGS. 7A and 7B indicates the test result for each of the cut-offfrequencies. Each of the curves in FIGS. 7A and 7B was obtained byplotting points corresponding to the false positive rate and the truepositive rate calculated for each of the thresholds.

For example, in the case where the threshold is zero (points at theright end of the graph), hands-on is always determined in the hands-onevents, and hands-on is always determined in the hands-off events. Thatis, both the true positive rate and the false positive rate are 100%. Inthe case where the threshold is always larger than the driver torque (inthis experiment, the torsion bar torque Ttb) (points at the left end ofthe graph), conversely, hands-off is always determined in the hands-onevents, and hands-off is always determined in the hands-off events. Thatis, both the true positive rate and the false positive rate are 0%.

In the case where the determination precision of the steering wheeloperation state determination unit is lowest, the graph has aninclination of 45 degrees, and the true positive rate and the falsepositive rate are equal to each other (an erroneous determination iscaused at a probability of ½). In the case where the determination.precision of the steering wheel operation state determination unit ishighest, on the other hand, the graph rises along the Y axis (verticalaxis), and extends in the direction of the X axis (horizontal axis) withthe Y-axis value asymptotic to 100%.

It is seen from FIGS. 7A and 7B that the determination precision of thesteering wheel operation state determination unit is lower for a badroad than for a paved road. It is also seen that the determinationprecision of the steering wheel operation state determination unit ishigher as the cut-off frequency of the low-pass filter is lower ineither road condition. If the cut-off frequency of the low-pass filteris low, however, the response of the hands-on/off determination is poor.From such viewpoints, the cut-off frequency of the low-pass filter ispreferably 3 [Hz] or more and 7 [Hz] or less, more preferably 4 [Hz] ormore and 6 [Hz] or less, most preferably 5 [Hz]. In the embodiment, thecut-off frequency of the low-pass filter 52 is set to 5 [Hz].

While an embodiment of the present invention has been described above,the present invention may be implemented in other embodiments, Forexample, in the embodiment discussed above, the driver torque estimationunit 51 uses the one-inertia-system model M1, and estimates the drivertorque Tsw using a driver torque estimation observer. However, thedriver torque estimation unit may use the two-inertia-system model M2illustrated in FIG. 4, and estimate the driver torque Tsw using a drivertorque estimation observer. Such a driver torque estimation unit 51Awill be described in detail,

The equation of state for the two-inertia-system model M2 is representedby the following formula (11).

$\begin{matrix}\left\{ \begin{matrix}\begin{matrix}{{\overset{.}{\hat{x}}e} = {{{Ae}\hat{x}e} + {{Beu}\; 1} + {{Le}\left( {y - \hat{y}} \right)}}} \\{= {{\left( {{Ae} - {LeCe}} \right)\hat{x}e} + {{Beu}\; 1} + {Ley}}}\end{matrix} \\{\hat{y} = {{Ce}\hat{x}e}}\end{matrix} \right. & (11)\end{matrix}$

“̂xe” (xe with a hat) in the formula (11) is a state variable vector, andis represented by the following formula (12).

$\begin{matrix}{{\hat{x}e} = \begin{bmatrix}{\theta \; {sw}} \\{\overset{.}{\theta}\; {sw}} \\{\theta \; {ww}} \\{\overset{.}{\theta}\; {ww}} \\{Tsw} \\{Tls}\end{bmatrix}} & (12)\end{matrix}$

“u1” in the formula (11) is an input vector, and is represented by thefollowing formula (13).

U1=T_(ma)  (13)

“y” in the formula (11) is an output vector (measured value), and isrepresented by the following formula (14). “̂y” in the formula (11) is anestimated value of the output vector.

$\begin{matrix}{y = \begin{bmatrix}{\theta \mspace{11mu} {ms}} \\{Ttb}\end{bmatrix}} & (14)\end{matrix}$

“Ae” in the formula (11) is a system matrix, and is represented by thefollowing formula (15).

$\begin{matrix}{{Ae} = \begin{bmatrix}0 & 1 & 0 & 0 & 0 & 0 \\\frac{- {ktb}}{Jsw} & \frac{- {ctb}}{Jsw} & \frac{ktb}{Jsw} & \frac{ctb}{Jsw} & \frac{1}{Jsw} & 0 \\0 & 0 & 0 & 1 & 0 & 0 \\\frac{ktb}{Jeq} & \frac{ctb}{Jeq} & \frac{- {ktb}}{Jeq} & \frac{- {ctb}}{Jeq} & 0 & \frac{1}{Jeq} \\0 & 0 & 0 & 0 & 0 & 0 \\0 & 0 & 0 & 0 & 0 & 0\end{bmatrix}} & (15)\end{matrix}$

“Be” in the formula (11) is an input matrix, and is represented by thefollowing formula (16).

$\begin{matrix}{{Be} = \begin{bmatrix}0 \\0 \\0 \\\frac{N}{Jeq} \\0 \\0\end{bmatrix}} & (16)\end{matrix}$

“Le” in the formula (11) is an observer gain matrix, and is representedby the following formula (17).

$\begin{matrix}{{Le} = \begin{bmatrix}{G\; 1} \\{G\; 2} \\{G\; 3} \\{G\; 4} \\{G\; 5} \\{G\; 6}\end{bmatrix}} & (17)\end{matrix}$

“G1”, “G2”, “G3”, “G4”, “G5”, and “G6” in the formula (17) are observergains, and have been set in advance.

“Ce” in the formula (11) is an output matrix, and is represented by thefollowing formula (18).

$\begin{matrix}{{Ce} = \begin{bmatrix}0 & 0 & N & 0 & 0 & 0 \\{ktb} & 0 & {- {ktb}} & 0 & 0 & 0\end{bmatrix}} & (18)\end{matrix}$

The driver torque estimation unit 51A computes the state variable vector̂xe on the basis of the equation of state represented by the formula(11). Consequently, an estimated value of the driver torque Tsw isobtained.

FIG. 8 is a block diagram illustrating the configuration of the drivertorque estimation unit 51A.

The driver torque estimation unit 51A includes a motor torquecomputation unit 81, a rotor angle computation unit 82, and a drivertorque estimation observer 102. The driver torque estimation observer102 includes an output vector input unit 83, an output matrix multiplier84, a first adder 85, a gain multiplier 86, an input matrix multiplier87, a system matrix multiplier 88, a second adder 89, an integrator 90,and a state variable vector output unit 91.

The motor torque computation unit 81 computes the motor torque Tmsgenerated by the electric motor 18 by multiplying the motor currentdetected by the current detection unit 32 by a torque coefficient of theelectric motor 18. The rotor angle computation unit 82 computes therotational angle (hereinafter referred to as a “rotor rotational angle”)θm of the output shaft of the electric motor 18 on the basis of theoutput signal from the rotational angle sensor 25.

The driver torque estimation observer 102 estimates the driver torqueTsw on the basis of the motor torque Tms computed by the motor torquecomputation unit 81, the rotor rotational angle θm computed by the rotorangle computation unit 82, and the torsion bar torque Ttb detected bythe torque sensor 11.

The rotor rotational angle θm computed by the rotor angle computationunit 82 and the torsion bar torque Ttb detected by the torque sensor 11are provided to the output vector input unit 83. The output vector inputunit 83 outputs the output vector y (see the formula (14)).

The output from the integrator 90 is the state variable vector ̂xe (seethe formula (12)). When the computation is started, an initial value ofthe state variable vector ̂xe is provided. The initial value of thestate variable vector ̂xe is zero, for example.

The system matrix multiplier 88 multiplies the state variable vector ̂xeby the system matrix Ae (see the formula (15)). The output matrixmultiplier 84 multiplies the state variable vector ̂xe by the outputmatrix Ce (see the formula (18)).

The first adder 85 subtracts the output (Ce·̂xe) from the output matrixmultiplier 84 from the output vector y. That is, the first adder 85computes the difference (y−̂y) between the output vector y and theestimated value of the output vector ̂y (=Ce·̂xe). The gain multiplier 86multiplies the output (y−̂y) from the first adder 85 by the observer gainLe (see the formula (17)).

The input matrix multiplier 87 multiplies the motor torque Tms (inputvector u1) computed by the motor torque computation unit 81 by the inputmatrix Be (see the formula (16)). The second adder 89 computes adifferential value d̂xe/dt of the state variable vector by adding theoutput (Be·u1) from the input matrix multiplier 87, the output (Ae·̂xe)from the system matrix multiplier 88, and the output (Le(y−̂y)) from thegain multiplier 86, The integrator 90 computes the state variable vector̂xe by integrating the output (d̂xe/dt) from the second adder 89. Thestate variable vector output unit 91 outputs the steering wheel angle(estimated value) θsw, the steering wheel angular velocity (estimatedvalue) dθsw/dt, the worm wheel angle θww (estimated value), the wormWheel angular velocity dθww/dt (estimated value), the driver torque Tsw(estimated value), and the load torque Tls (estimated value) on thebasis of the state variable vector ̂xe. The driver torque (estimatedvalue) Tsw output from the state variable vector output unit 91 isprovided to the low-pass filter 52 (see FIG. 3).

In the embodiment discussed above, the electric motor 18 is athree-phase brushless motor, However, the electric motor 18 may be abrushed direct-current (DC) motor.

What is claimed is:
 1. A vehicle steering device comprising: a steeringwheel; an input shaft to which the steering wheel is coupled; an outputshaft coupled to the input shaft via a torsion bar; an electric motorcoupled to the output shaft via a speed reduction mechanism; a torquedetector that detects torsion bar torque applied to the torsion bar; arotational angle detector that detects a rotational angle of theelectric motor; and a steering wheel operation state determinationdevice, wherein: the steering wheel operation state determination deviceincludes: a driver torque estimation observer that estimates drivertorque applied to the steering wheel by a driver using a detected valuethat includes at least the torsion bar torque detected by the torquedetector and the rotational angle of the electric motor detected by therotational angle detector; a low-pass filter that performs a low-passfiltering process on the driver torque estimated by the driver torqueestimation observer; and a hands-on/off determination unit thatdetermines whether in a hands-on state or a hands-off state on a basisof the driver torque after being subjected to the low-pass filteringprocess by the low-pass filter.
 2. The vehicle steering device accordingto claim 1, wherein: the speed reduction mechanism includes a worm gearrotationally driven by the electric motor, and a worm wheel coupled soas to be rotatable together with the output shaft and rotationallydriven by the worm gear; and the driver torque estimation observer isconfigured to estimate the driver torque on a basis of a rotationalangle and an angular velocity of the worm wheel computed on a basis ofthe rotational angle of the electric motor detected by the rotationalangle detector, and the torsion bar torque.
 3. The vehicle steeringdevice according to claim 1, further comprising: a current detectionunit that detects a motor current that flows through the electric motor,wherein the driver torque estimation observer is configured to estimatethe driver torque on a basis of a motor torque of the electric motorcomputed on a basis of the current detected by the current detectionunit, the rotational angle of the electric motor detected by therotational angle detector, and the torsion bar torque.
 4. The vehiclesteering device according to claim 1, wherein a cut-off frequency of thelow-pass filter is 4 [Hz] or more and 6 [Hz] or less,